1. Field of the Invention
The present invention relates generally to an exhaust system for an outboard motor, and more particularly to an improved exhaust system for an outboard motor that has an exhaust discharge port at a portion of a housing unit of the outboard motor.
2. Description of Related Art
An outboard motor typically includes a housing unit that can be mounted on an associated watercraft and an engine disposed above the housing unit. The outboard motor also includes an exhaust system to discharge exhaust gases from one or more combustion chambers of the engine to a location outside of the motor. Typically, an underwater exhaust discharge port is formed at a lowermost section of the housing unit so that the exhaust gases are discharged to a body of water surrounding the outboard motor when the outboard motor is mounted to an associated watercraft. An above-water exhaust discharge port also is formed at a higher section of the housing unit to discharge exhaust gases under idle condition of the engine.
The outboard motor normally employs a propeller as a propulsion device powered by the engine. A crankshaft of the engine drives a driveshaft and a propulsion shaft coupled with the driveshaft. The propulsion shaft then drives the propeller. A transmission also is employed to change a rotational direction of the propeller among forward, neutral and reverse.
When an operator of the outboard motor shifts the transmission, for example, to the reverse direction from the forward direction, the inertia of water flow by the propeller can cause the impeller to continue to rotate in the forward direction even after the transmission has been shifted into reverse. As such, the impeller can rotate the crankshaft inversely through the driveshaft and the propeller shaft. An engine control device such as, for example, an ECU (electronic control unit) recognizes the reverse rotation of the crankshaft and controls the engine to stop. However, for a moment before the engine stops, the exhaust system can generate negative pressure. For example, if the crankshaft is rotated in the reverse direction, driving a piston downwardly while an exhaust valve is open, air will be drawn into the engine through the exhaust system. Because of this negative pressure, the underwater and above-water ports can draw water or air containing water, respectively, into the exhaust system. The water can reach the engine and can cause rust or corrosion of the engine. Particularly, if the water contains salt, the corrosion can ruin the engine faster.